(1) Field of the Invention
The present invention relates to a toner for the electrophotography. More particularly, the present invention relates to a toner for the electrophotography having a broad fixing temperature range and having a high fixing ratio and an excellent copying resistance, and also to a process for the preparation of this toner.
(2) Description of the Related Art
In the dry electrophotographic process, fixation of a toner to a copying paper is generally accomplished by heat fixation, and this heat fixation is effected by supplying a copying paper having a toner image transferred from a photosensitive plate between a pair of rolls, at least one of which is heated. A toner formed by dispersing a colorant, a charge controlling agent, a release agent and other additives into a binder resin and adjusting the particle size to an appropriate level is used as the toner for the heat fixation.
In general, the fixing capacity of the toner depends mainly on the molecular weight distribution of the binder resin and the kind of the release agent such as a wax. Namely, if the molecular weight of the binder resin is low, the fixing temperature is generally low, and at a high temperature, there occurs high-temperature offset. On the other hand, if the molecular weight is high, the fixing temperature becomes high, and low-temperature offset or insufficient fixation tends to occur.
Accordingly, Japanese Examined Patent Publication No. 57-111543 proposes a process in which a binder resin having molecular weight distribution peaks at a molecular weight of 5000 to 80000 and a molecular weight of 100000 to 200000 is used to prevent high-temperature offset and low-temperature offset.
Since occurrence of the offset phenomenon can be prevented by improving the release property of the toner, various proposals have been made on selection of the kind and amount incorporated of the release agent such as a wax.
The above-mentioned prior art technique is excellent in that necessary and minimum fixation can be carried out at a specific fixing temperature without occurrence of any special trouble. However, in the case where it is desired to further improve the fixing ratio or it is intended to perform sufficient fixation even at a low temperature, it is necessary that the amount of the component having the molecular weight distribution on the low molecular weight side should be increased and the molecular weight on the low molecular weight side should be further lowered.
In the toner of the above-mentioned prior art technique, if the molecular weight of the peak on the low molecular weight side is lower than 15000, the fixing ratio is drastically improved, but high-temperature offset often occurs, the copying resistance (the number of obtainable copies) is degraded and contamination of the rolls becomes conspicuous. If the amount incorporated of a release agent such as a wax is increased for overcoming this disadvantage, blocking of the toner is caused, and the chargeability, transferability and fixing property are adversely influenced.